Astrophile: Attack of the mystery green blobs

Astrophile is our weekly column covering curious cosmic objects, from within the solar system to the furthest reaches of the multiverse

Object type: Glowing gas cloud Mass: Billions of times the mass of the sun

Imagine making a night-time trek to a remote stretch of desert, far from any sign of civilisation. You crest a hill and are astonished to find a building ablaze with artificial lighting.

That is a little like the puzzlement that greeted the discovery of Hanny's Voorwerp, a curious gas cloud found floating in intergalactic space in 2007. It is brighter than 30,000 suns but has no obvious power source. Now, 19 similar clouds have been discovered, all glowing apparently without internal power.

The clouds were probably energised by nearby monster black holes that had blasted them with intense radiation. This link to huge black holes is exciting because it means the clouds could be an excellent new way to probe the growth and feeding habits of these inscrutable behemoths.

Hanny's Voorwerp was discovered by the schoolteacher Hanny van Arkel while she was classifying galaxies as a volunteer for the Galaxy Zoo citizen science project. She noticed a weird blob that appeared intensely blue in the false-colour image she was examining and emailed the Galaxy Zoo researchers about it. Voorwerp means "thing" in Dutch, Van Arkel's native language.

Monster suspect

Intrigued, the researchers scheduled new telescopic observations of the object. "It became clear quickly just how special it was," says William Keel of the University of Alabama in Tuscaloosa, a member of the observation team, which was led by Chris Lintott of the University of Oxford.

The object's light spectrum shows that its glow comes from oxygen that has been ionised – stripped of some of its electrons – along with other ionised elements, making its true hue a greenish colour. It would take a huge amount of energy to ionise all this gas, but there was no hint of a source. Radiation from hot young stars could account for ionised oxygen in the cloud, but not the ionised neon: neon doesn't shine in the ultraviolet, as seen in the cloud, without lots of X-rays hitting it.

That suggested a monster black hole was involved. Most galaxies are thought to host one in their cores and in many cases matter spiralling into the black holes produces huge amounts of X-rays.

A galaxy called IC 2497 lies about 45,000 to 70,000 light years from the glowing cloud, and a black hole at its core could easily blast Hanny's Voorwerp with X-rays. But there's a catch. IC 2497's core shows no sign of emitting X-rays.

Brutal binges

In 2008, the team concluded that less than 100,000 years before IC 2497 became the galaxy we see today , its black hole was gulping down a big meal and sending out a torrent of X-rays. Because it takes time for the X-rays to reach the cloud, some of them were still arriving and making it glow when it emitted the light Van Arkel saw, even though the black hole was by then quiet.

That's a rare bit of evidence of how much black-hole feeding can vary over tens of thousands of years. Researchers are keen to understand the feeding habits of black holes because such binges, called accretion events, have an enormous effect on their surroundings, shutting off galaxy growth by heating and expelling the gas needed to form new stars.

No freak

But it was not clear how representative Hanny's Voorwerp was of black-hole behaviour. Now, professional researchers and Galaxy Zoo volunteers working together have found 19 similar objects – glowing gas clouds near galaxies whose black holes appear quiet but probably blasted the clouds in the past.

Three-quarters of the newly discovered clouds have a nearby galaxy that is interacting or merging with another galaxy, according to the new study, which Keel is leading. That fits with the black-hole-blast explanation, because such encounters tend to shake loose gas clouds that then stray into intergalactic space – providing targets to be illuminated by black hole X-rays.

It also shows Hanny's Voorwerp is not a freak. All over the universe, black holes are apparently firing broadsides at their surroundings, then quickly quietening down and fading from view, like flashing lights.

"If you could watch the cosmic movie and speed it up, it would be like a Christmas tree, with these [X-ray blasts] popping off in one galaxy, then the next," says Keel.

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